Change parameters to arguments in hacking/validate

We recently changed the notion of parameters to arguments [1].
Forgot to update hacking/validate.

[1]: https://review.openstack.org/#/c/610389/

Change-Id: I7fc7d16ed68fe9d5f1769eca75f62f99004de0e9

hacking/validate.py

@@ -23,7 +23,7 @@ from ara.clients.offline import AraOfflineClient
 
 
 def validate_playbook(playbook):
-    assert len(playbook['parameters']) == 40
+    assert len(playbook['arguments']) == 40
     assert 'hacking/test-playbook.yml' in playbook['file']['path']
     assert len(playbook['files']) == 9
     assert playbook['completed']